1. Field of the Invention
The present invention relates to the thermal pressure forming of precut plastic blanks into retortable containers, in particular, while the precut plastic blanks are substantially at their melt phase.
2. Description of the Related Art
Processes and devices for producing plastic material of either a monolayer or laminated multilayers providing barriers to both gas and moisture are old and well-known in the food packaging industry. Conventionally, such processes include the steps of extruding sheets of plastic material, cutting blanks or billets from such sheets, heating the material to a desired temperature range, and pressure forming the material into food or beverage containers. Thereafter, the containers are sealed so that the contents may be stored for extended periods of time without spoilage.
Exemplary prior art methods and apparatuses were developed by the Shell Oil Company and patented between 1970 and 1986. Generally, this technology is known as solid-phase pressure forming (SPPF) because the container is formed under pressure while the plastic blank is hot but still in its solid (unmelted) phase. Some aspects of this so-called SPPF technology are protected by the following patents:
______________________________________ U.S. PAT. NO. INVENTOR(S) ISSUE DATE ______________________________________ 3,499,188 Johnson March 10, 1970 3,502,310 Coffman March 24, 1970 3,532,786 Coffman October 6, 1970 3,546,746 Johnson December 15, 1970 3,606,958 Coffman September 21, 1971 3,608,058 Coffman September 21, 1971 3,642,415 Johnson February 15, 1972 3,684,258 Coffman et al August 15, 1972 3,733,159 Coffman May 15, 1973 3,757,718 Johnson September 11, 1973 3,859,028 Van der Gaag et al January 7, 1975 4,172,875 Beijen et al October 30, 1979 4,229,405 Coffman October 21, 1980 4,563,325 Coffman January 7, 1986 ______________________________________
The preferred method and apparatus used in SPPF technology extrudes plastic in long, thin (generally 0.030 to 0.100 inches) sheets for subsequent processing. However, SPPF technology is limited to forming containers while the plastic blanks are still in their solid phase, i.e. at temperatures below the crystalline melt point of the material.
The solid to liquid (melt) transition takes place over a temperature range for each particular polymer. A calorimeter will measure the heat input required to raise the temperature of a sample of the material. The latent heat required to melt the material can be measured and it will be noted that melting begins at a certain temperature and continues over a wide temperature range. A majority of the melting will occur at a particular temperature noted by a peak on the Differential Scanning Calorimeter curve. This temperature is commonly referred to as the "melting point". The range for polypropylene may be from 240.degree. F. to 340.degree. F. with the "melt point" peak at 330.degree. F. SPPF technology is practiced normally at 320.degree. F. to 328.degree. F.
Heretofore, it has not been practicable to package and process many commodities, particularly those containing bulk solids, in thermoformed plastic containers because the containers could not withstand the combination of pressures and temperatures in a retort chamber without undergoing significant distortion.
SPPF technology requires that the temperature of the plastic materials during the thermal pressure forming stages remain in a domain generally 5 to 20 degrees Fahrenheit (.degree.F.) below the melting point of the plastic. Thermal pressure forming of the plastic material in this temperature range insures that the material is soft enough to be formed into containers but that such plastic material does not reach the melt phase, at which point severe handling problems start to occur. Such containers are generally not retortable.
Other exemplary prior art methods and apparatuses were developed by the Dow Chemical Company and patented between 1973 and 1977. This technology is generally known as the Scrapless Forming Process (SFP) in which containers are formed from square "chips" cut from an extruded plastic sheet. The square chips are first forged into circular discs (for circular containers), which discs are then formed under pressure into containers. No scrap is generated in the process steps from cutting the chip to forming the container. Some aspects of this so-called SFP technology are protected by the following patents:
______________________________________ U.S. PAT. NO. INVENTOR(S) ISSUE DATE ______________________________________ 3,739,052 Ayres et al June 12, 1973 3,947,204 Ayres et al March 30, 1976 3,995,763 Ayres et al December 7, 1976 4,005,967 Ayres et al February 1, 1977 4,014,965 Stube et al March 29, 1977 ______________________________________
Containers produced using SFP technology have also been found to be unsatisfactory for retorting because here also the forming process is essentially the same as SPPF technology in that stress relaxation, effected by the elevated temperatures and pressures of a commercial canning retorter, will cause unacceptable deformation of containers. Furthermore, SFP technology requires that the billets or chips be lubricated prior to the forging step. Though necessary for the Scrapless Forming Process, the use of lubricants is undesirable for containers intended for the storage of human foodstuffs.
Thus, it remains a problem in the prior art, particularly in SPPF and SFP technologies, to produce a plastic container which can be used in a commercial canning retort process without becoming distorted to an undesirable extent. The prior art specifically acknowledges a marked tendency of melt-phase plastic billets or blanks to deform during heating and the serious handling problems that occur if the plastic blanks are heated above their melting points.